Discovery of a new group of double-periodic RR Lyrae stars in the OGLE-IV photometry

TL;DR

This paper reports the discovery of a new group of double-periodic RR Lyrae stars with a unique period ratio, identified through OGLE-IV and Kepler data, revealing complex pulsation modes.

Contribution

The study uncovers a previously unknown class of RR Lyrae stars exhibiting a distinct double-periodic behavior with a specific period ratio.

Findings

11 stars with additional low-amplitude periods detected in OGLE-IV data

1 star with similar variability identified in Kepler data

Period ratio around 0.686, longer than expected for fundamental mode

Abstract

We report the discovery of a new group of double-periodic RR Lyrae stars from the analysis of the OGLE-IV Galactic bulge photometry. In 11 stars identified in the OGLE catalog as first overtone pulsators (RRc stars) we detect additional longer period variability of low amplitude, in the mmag regime. One additional star of the same type is identified in a published analysis of the Kepler space photometry. The period ratio between the shorter first overtone period and a new, longer period lies in a narrow range around 0.686. Thus, the additional period is longer than the expected period of the undetected radial fundamental mode. The obvious conclusion that addition periodicity corresponds to a gravity or a mixed mode faces difficulties, however.